Flash setting, 4.26

When the kiln material is cooled it forms into crystallized clinkers. These are rather large irregular pieces of the solidified cement material. These clinkers are ground and a small amount of gypsum is added (usually about 1.5 to 3%). The gypsum prevents flash setting of the cement and also controls free CaO. This final cement product is sampled, analyzed and stored. The actual commercial cement is usually a hlend of several different cements. This blending ensures a consistent product. 

Adhesion Adhesion of the plastic dry-mix silica fume shotcrete, particularly in wet areas, is substantially improved. For example, adhesion in areas such as locks, dry docks, tunnels, and even leaking structures, can only be achieved through the use of high concentrations of shotcrete accelerators, to create a flash-setting condition, a process that is detrimental to the long-term durability of the hardened shotcrete. Silica fume has been found to promote excellent adhesion in such conditions with minimal or no accelerator addition. 

The firing pins cause the primers to ignite and the resulting flash sets off the delay elements, which are followed by relay elements, detonators, auxiliary booster lead-ins, booster leads, booster and main charge of the bomb (Ref 51a, pp 4-44 to 4-47)... 

Water-reducing agents, also called plasticizers, allow a given degree of workability to be achieved at a lower w/c ratio. Conventional water reducers allow w/c to be decreased by 5-15%. To achieve greater decreases, they would have to be used at concentrations that would also cause excessive retardation, excessive air entrainment or flash setting. More powerful water reducers, called superplasticizers, allow w/c to be decreased by up to about 30 /o these are considered in Section 11.4.2. 

The fact that the water-reducing effects of superplasticizers are similar to those of conventional water reducers used in similar concentrations suggests that the modes of action are similar, and that the essential difference is that the properties that limit the concentrations in which conventional water reducers can be used are weaker or absent in superplastizers. These properties comprise retardation, air entrainment and in some cases flash setting. 

If the formation has openings too large for the cement to plug them, much of the cement, pumped down the hole, may be lost. In such cases, it is necessary to first plug these openings by means of flash-setting cements. CBS formulations can be designed to flash set... 

When the CBS slurry is pumped down a borehole and as it travels through the borehole, its temperature will increase with depth. The higher temperature wiU, in turn, increase the solubility of the oxide components in the slurry. If the solubility of the oxides becomes too high, the slurry may flash set within the borehole prematurely and clog it. To avoid this situation and to allow free flow of the slurry, one must control the solubility of the oxides. Alternatively, one may use components that have their maximum solubility only at the... 

Setting in hydrocarbon environment Unaffected by CO2 Flash-sets by carbonation CBS is most useful in gas hydrate regions... 

Note The bullet should be slanted downward with a I metal tube extending from It, blocked at the end with paper, so that the flash sets the charge off at the bottom middle. Better yet, put the detonator below the charge so the bullet rests at the bottom. 

As it has been mentioned, the hydration of CjA has a decisive impact on the rheological properties of fresh paste. The high rate of reaction with water leads to the saturation of solution with aluminate and calcium ions and as a consequence to the crystallization of C AHj. This corresponds to the quick stiffening of paste, determined as flash set. All the substances modifying the rate of CjA reaction with water by adsorption on the surface of this phase or by the change of the ions concentration in the liquid phase will have a great impact on the rheological properties of paste. 

According to Locher and Richartz [18], as well as Sprung [44] the formation of syngenite can be also the reason of false set. This phase is produced in cement pastes with higher amount of K2SO4 and hemihydrate. The formation of syngenite causes not only flash set but also leads to the consumption of sulphates, and further to the rapid C3A reaction with water. As a consequence, the stiffening of paste takes place [45]. 

Owing to the presence of elevated amounts of C3A in low-iron clinkers it is advisable to introduce the SO3 required for set control in the form of gypsum, rather than anhydrite, as it has a faster dissolution rate and thus may be more effective in preventing flash setting. 

It has long been recognized that, even in the absence of calcium sulfate, flash setting of cement may be prevented and a cement suspension of flowing consistency obtained by combining finely groimd Portland clinker with an alkali metal carbonate or hydrogen... 

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